The present invention relates to light strings such as are used for holiday lighting, and in particular to mechanical shunts for passing electrical current to the next light in the string if the bulb is loose or missing.
Strings of lights are typically wired electrically in series. Consequently, when one light in the string burns out or is removed, all the lights in the string go out. Determining which light needs to be replaced is tedious. If the string has 50 or more lights and the string is attached to a Christmas tree, finding the burned out or missing bulb can be very tedious.
For a number of years, this problem has been solved, or at least avoided, by the use of shunts that allow current to pass directly between the terminals of the defective lamp, bypassing the missing or defective bulb filament. Passing electrical current from one lamp to the next regardless of the condition of the bulb in any individual lamp allows the remaining lamps to continue to operate.
Shunts are typically found in two places in prior art lamps, namely, in the glass globe and in the socket. The shunts inside the glass globe are typically made of wire wrapped around the conductive elements (called Dumet wires). When the filament fails, the oxide coating on the wires that theretofore prevented direct conduction of electricity is burned off and the coil welds itself to the Dumet wires, thereby providing a new electrically conductive path for passing the electrical current.
Of the shunts that are located in the socket, there are two types, namely, solid state shunts and mechanical shunts. Among the mechanical shunts, for example, there is a set of spring contact terminals that is the subject of U.S. Pat. No. 6,257,740. These spring contacts are pushed apart when the lamp base is inserted into the socket and spring back together when the base is removed, thereby allowing the current to pass from one terminal to the other directly. This type is strictly for use when the bulb (and its base) is removed and does not address the issue of a burned out bulb. This type of shunt works well and has enjoyed commercial success.
Another mechanical shunt is disclosed in U.S. Pat. No. 7,253,556, which is invented by the present inventor and is commonly owned by applicant. This mechanical shunt is a nearly horizontal flat strip of metal held in place between the two electrical terminals in a light socket by a shunt holder. The ends of the shunt extend laterally and slightly downwardly to engage the electrical terminals mounted to the socket wall. When the lamp base, which is hollow, is inserted into the socket, the shunt holder together with its shunt is received inside the hollow base, and, as the shunt enters the base, its lateral ends are bent down and away from the electrical terminals on the socket wall, thereby allowing electrical current to pass to and through the Dumet wires and thence to the filament in the bulb rather than directly through the shunt between the electrical terminals.
U.S. Pat. No. 6,609,814 issued to Ahroni teaches an asymmetrical mechanical shunt composed of two portions that are nearly co-planar with the bottom of one portion contacting the top of the other portion. Each of these two portions of the shunt are in electrical contact with one of the electrical terminals mounted on the wall of his socket. When a lamp holder is inserted into the socket, it presses a first of the two portions down and thus out of engagement and electrical connection with the second of the two portions of the shunt. The first portion resiliently resumes electrical contact with the second portion once it is freed to resilient spring back into engagement by the removal of the lamp holder from the socket.
Manufacturers of miniature lamps are concerned with cost of materials and labor. Small lamps are assembled largely by hand. Accordingly, small components that need to be added to the socket increase labor costs as well as material costs. While individually their cost is trivial, the cumulative cost for materials and labor of the huge number of miniature lights made and sold every year in a competitive marketplace collectively is substantial. Correspondingly, even small changes that, for example, reduce material requirements, simplify manufacturing, or improve safety or reliability, make a huge difference in the costs to manufacture.
Another concern regarding shunts in light sockets is the requirement of most shunts to receive only one type of holder, namely, one that works with the shunt, typically to activate it or manipulate it in some fashion. In many applications, the user may wish to place a special decorative device in the socket for power to illuminate it or cause some small component of the ornament to move. These include, for example, the so-called pigtail ornaments, such as that described in U.S. Pat. No. 6,764,205, issued to Peloquin. This type of ornament contains a light that is connected to a cord (or “pigtail”) the distal end of which is inserted into a socket of a light string for its source of electrical power for the light in the ornament. Because the connector on the end of the pigtail does not have and special structure needed to activate the mechanical shunt, these types of ornaments are then operating without the shunt protection or the distal end will not fit properly into the socket. This presents a dilemma for the user. If such an ornament is used, the shunt will often be inoperable or will always be operable, even when the ornament is in the socket.
Finally, particularly in the case of lights used out of doors, when temperatures increases during the day cause the sockets of lights strings to expand, the lamp holders will tend to come out of their sockets, particularly if the lamps are attached to a support or tree so that they are hanging down. Under these circumstances, a mechanical shunt becomes all the more important to keep the remaining lamps burning and to allow those responsible for the light to quickly find the missing lamps in the string. Accordingly, there remains a need for a better mechanical shunt for use in the sockets of the lamps of light strings.